Vapor compression systems (VCS) move thermal energy between a low temperature environment and a high temperature environment in order to perform cooling or heating operations and to improve comfort of the occupants. For example, heat can be moved from an indoor space to an outdoor space in order to lower the indoor temperature in a cooling operation, or heat can be moved from an outdoor space to an indoor space in order to raise the indoor temperature in a heating operation.
The heat load, or rate at which the thermal energy is moved into a space (e.g., by hot air passing into a building) is generally not directly measured, but its effect is detected as changes in the indoor space temperature or zone temperature. In order to control the zone temperature, the operations of the VCS modulates the cooling or heating capacity provided by the system to counteract the load such that the zone temperature is near a desired zone temperature. The thermal capacity of a heat exchanger is the rate at which the thermal energy is accepted or rejected by a heat exchanger.
A multi-zone vapor compression system (MZ-VCS) includes a single compressor connected to a multiple heat exchangers arranged in one or more indoor zones. The heating or cooling capacity of such indoor heat exchangers is modulated by duty cycling each heat exchanger between “ON” and “OFF” modes of the operation. The heat exchanger is OFF when an inlet valve that controls refrigerant flow is closed or alternatively, the compressor that pumps refrigerant through the system is stopped, so that no cooling or heating is performed by the heat exchanger. The heat exchanger is ON when an inlet valve is opened and the compressor is operating so that the heat exchangers in the indoor zones operate at their full thermal capacity. A controller decides how to alternate between the modes based on a difference between the zone temperature and desired zone temperature.
However, the act of switching heat exchangers ON and OFF, especially in MZ-VCS where the zone heat exchangers can switch ON and OFF independently from each other, result in persistent periodic variations in the outputs of the system, such as zone temperatures and heat exchanger temperatures, that are known to be inefficient and reduce occupant comfort. Accordingly, there is a need in the art for a control system and method to smoothly control the thermal capacity of heat exchangers, such as the heat exchangers of MZ-VCS.
The smooth control the thermal capacity of heat exchangers is even more challenging for the heat exchangers designed with multiple parallel refrigerant flow paths splitting the flow of the refrigerant. Splitting the refrigerant flow within a heat exchanger decrease the flow rates of the refrigerant mass within individual paths allowing longer transit time for refrigerant within the heat exchanger, hence, providing more opportunity for the heat exchange and thereby increasing system efficiency.
However, it is commonly recognized that evenly distributing refrigerant among the multiple paths of a multi-path heat exchanger is difficult to arrange. For example, the theoretically equally split refrigerant flows more into one path than to the other path causing the complications in thermal management of the heat exchangers. A number of conventional methods aim to address the problem of uneven distribution of the refrigerant.
For example, one method uses a specially designed header pipe distributing refrigerant to the multiple paths so that the refrigerant in each path is uniform, see e.g., U.S. 2011/0017438 and U.S. 2013/0312944. Another method uses a complicated distributor including a header pipe and a multiplicity of controllable valves to achieve even refrigerant distribution by actively metering the amount of refrigerant allowed on each path, see, e.g., U.S. Pat. No. 8,794,028 and U.S. Pat. No. 8,689,582. However, all those methods increase the cost of the VCS and do not always achieve an optimal result.
Accordingly, there is a need in the art for a low cost method for controlling refrigerant flow in multi-path heat exchangers that does not require additional expensive distributors.